energies Article Study on the Effects of Evaporation and Condensation on the Underfloor Space of Japanese Detached Houses Using CFD Analysis Wonseok Oh 1,* and Shinsuke Kato 2 1 Graduate School of Engineering, Department of Architecture, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 2 Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-3-5452-6430 Academic Editor: Bjørn Hjertager Received: 27 April 2017; Accepted: 8 June 2017; Published: 13 June 2017 Abstract: The purpose of this study is to determine the effects of evaporation and condensation on the underfloor space of Japanese detached houses. In this underfloor space, natural ventilation is applied. A typical Japanese wooden detached house is raised 0.3–0.5 m over an underfloor space made of concrete. The bottom of the underfloor space is usually paved with concrete, and the ceiling which is directly underneath the indoor occupant zone is made of wood. Computational fluid dynamics (CFD) analysis is applied to calculate the rates of the evaporation and condensation generated inside the underfloor under two conditions, namely, a constant (fixed) outdoor environmental condition and a fluctuating environmental condition. In the constant condition, we verified the effects of the outdoor humidity, ventilation rate, and ratio of wetted surface (RWS, !) on the evaporation and condensation inside the underfloor space. In this condition, the rate of evaporation and condensation was quantified considering the varying outdoor humidity between 0 to 100%, and the RWS (! = 1 or 0). In addition, the influence of the different ventilation rates at 1.0 m/s for normal and 0.05 m/s for stagnant wind velocities were confirmed. Under fluctuating environmental conditions, the outdoor conditions change for 24 h, so the RWS varies. Therefore, the rate of evaporation and condensation, the amount of the condensed water, and the area of condensation were confirmed. The results were as follows: with a high airflow rate on the underfloor space, the evaporation and condensation phenomenon occurs continuously and is easily affected by outdoor humidity, while under low airflow rate conditions, only the condensation appeared steadily. If the wind velocity is strong, the convective mass transfer on a surface becomes large. In a condition of the outdoor humidity and the airflow rate on underfloor are high, condensation mainly occurs in a corner of the underfloor space due to high evaporation by convection in the mainstream of the airflow. By contrast, when the airflow rate is low, condensation occurs along the air stream. Accordingly, this information could be employed as design considerations for the underfloor space at the architectural design stage. Keywords: underfloor ventilation; evaporation and condensation; computational fluid dynamics (CFD); ratio of wetted surface (RWS) 1. Introduction The aim of ventilation of the underfloor space is to prevent moisture generated in the ground from entering the living area and to improve the heating and cooling efficiency of the building, given the large heat capacity of the ground. Moisture is the main problem in wooden buildings, as it causes rotting of Energies 2017, 10, 798; doi:10.3390/en10060798 www.mdpi.com/journal/energies Energies 2017, 10, 798 2 of 20 Energies 2017, 10, 798 2 of 21 the wood and an unpleasant smell inside the building. Even so, the underfloor space is generally ignored bygenerally the residents, ignored as it isby not the only residents, inconspicuous, as it is butnot alsoonly difficult inconspicuous to clean., Generally,but also difficult moisture to originates clean. fromGenerally, the ground moisture (soil), originates which absorbs from waterthe ground after rain, (soil), or fromwhich groundwater, absorbs water and after enters rain, the or building from throughgroundwater, the ventilation and enters openings. the building It is through important the ventilation to block the openings. source It of is the important moisture to block in order the to controlsource the of humidity the moisture level in and order to prevent to control the generationthe humidity of condensationlevel and to prevent inside the the underfloor generation space. of Therefore,condensation it is inside crucial the not underfloor only to preventspace. Therefore, ground it moisture is crucial fromnot only entering, to prevent but ground also to moisture eliminate anyfrom moisture entering forming, but also inside to eliminate from rainwater any moisture or high forming humidity. inside from Since rainwater these phenomena or high humi influencedity. theSince health these of the phenomena occupants influence and the durabilitythe health ofof the the buildings, occupants methods, and the durability such as paving of the of buildings, the ground ormethods, mechanical such ventilation as paving of are the applied ground occasionally.or mechanical However,ventilation itare is applied useful tooccasionally. predict the However, climate of theit underfloor is useful to space predict in the attemptclimate of to the control underfloor the humidity space in and the to attempt apply anto appropriatecontrol the humidity method duringand construction,to apply an asappropriate installing method such alternative during construction, methods could as installing be costly. such alternative methods could be costly.In previous research, the relative humidity, temperature, and pressure inside the underfloor space were measured,In previous the research, ground moisturethe relative evaporation humidity, temperature, rate was calculated, and pressure and subsequently inside the underfloor the effects ofspace natural were and measured, mechanical the ventilationground moisture were evaporation confirmed [rate1]. was The calculated, results showed and subsequently that the average the effects of natural and mechanical ventilation were confirmed [1]. The results showed that the average value of moisture evaporation with unpaved ground was 3.6 g h−1 m−2 in a naturally ventilated value of moisture evaporation with unpaved ground was 3.6 g h−1 m−2 in a naturally ventilated condition and 5.7 g h−1 m−2 in a mechanically ventilated condition. Research also indicated that condition and 5.7 g h−1 m−2 in a mechanically ventilated condition. Research also indicated that the the optimum air change rate on the underfloor space was 1–3 air change rate per hour (ACH) optimum air change rate on the underfloor space was 1–3 air change rate per hour (ACH) throughout throughout the year [2]. Experiments have also been conducted with benzene in the underfloor the year [2]. Experiments have also been conducted with benzene in the underfloor space to identify spacethe potential to identify health the potentialrisks associated health with risks toxic associated soil vapors with penetrating toxic soil into vapors the penetratingliving spaces into[3,4]. the livingThese spaces studies [3 ,4employed]. These studiespoint measurements. employed point However, measurements. the internal However, humidity the was internal excluded humidity as a wasfactor excluded in the asresearch. a factor Several in the research.studies have Several attempted studies to haveconfirm attempted the presence to confirm of internal the presencehumidity of internalby employing humidity computational by employing fluid computational dynamics (CFD) fluid to dynamics determine (CFD) the distribution to determine of thesuch distribution internal of suchhumidity. internal However, humidity. no research However, has no been research conducted has been on the conducted internal condensation on the internal of the condensation underfloor of thespace underfloor. In the space. current In thestudy, current we study,attempt we to attempt determine to determine the effects the of effects internal of internal condensation condensation and andevaporation evaporation and and the the probable probable area area where where condensation condensation would would occur occur relevant relevant to tothe the natural natural ventilation.ventilation. Research Research on on such such aspects aspects has has notnot beenbeen conducted before. before. A JapaneseA Japanese detached detached house house can can be be simplysimply illustratedillustrated as as in in Figure Figure 11.. The The height height of of the the underfloor underfloor spacespace is is 0.3–0.5 0.3–0.5 m m from from ground ground level. level. AsAs openingsopenings are dispersed dispersed around around the the underfloor underfloor space space for for naturalnatural ventilation, ventilation, outdoor outdoor moist moist air flows air flows into the into underfloor the underfloor space, which space, gives which rise togives condensation rise to andcondensation evaporation. and The evaporation. bottom of theThe underfloor bottom of the space underfloor is usually space paved is usually with concrete paved with and theconcrete ceiling, whichand isthe directly ceiling, underneathwhich is directly the indoor underneath occupant the indoor zone, isoccupant made of zone, wood. is made of wood. DWELLING Underfloor Space Evaporation Moist Air Condensation GROUND Figure 1. Underfloor ventilation in a Japanese detached house. Figure 1. Underfloor ventilation in a Japanese detached house. Energies 2017, 10, 798 3 of 20 Energies 2017, 10, 798 3 of 21 ThereThere are are two main causes of condensation on on the underfloor underfloor environment, environment, namely, namely, moisture derivingderiving